1. Field of the Invention
The present invention relates to a method and a device used for simulating an impulse-type CRT display, and more particularly, to a method and a device used for simulating an impulse-type CRT display using a liquid crystal display (LCD).
2. The Prior Arts
In recent years, the technology and device of a liquid crystal display (LCD) have been very popular and widely used for the consumer electronic products, especially for video products, for example, televisions, computers, displays, telephone handsets, personal data assistants (PDA), and the like. The varieties of the products are enormous, so as to stimulate the tremendous rapid progress of the technology of the liquid crystal display and its direction of development is in agreement with the requirement of the future trend of development of electronic products toward the features of light weight, thin thickness, short length, small size, low power consumption, and low heat dissipation, etc.
Presently, televisions and display devices made with the technology of the liquid crystal display have been produced in large quantities, to replace the televisions and display devices made with the conventional CRT. However, in the liquid crystal display technology of the present days, there still exist drawbacks and limitations, which must be overcome and improved.
With regard to the CRT display, it utilizes the “impulse type” display method. It produces light emissions by means of irradiating a single electron beam on the pixels coated with fluorescence materials. However, as shown in curve (a) in FIG. 1, the pixel only produces the emission of light in a brief instant of time in each frame period. Therefore, it almost seems to have no image overlapping phenomenon to be noticed for the images displayed between the frames.
However, for the LCD display, it utilizes the “hold type” display due to the intrinsic property of the LCD material. It produces the image display through the optical response (namely, the gray level response) by means of applying driving voltages on the LCD material. Nevertheless, due to the limitation of the intrinsic property of the liquid crystal material, the image it displays occupies the predominant portion of time of that frame as shown in curve (c) in FIG. 1. And for every time its image changes, its luminance (or brightness) also changes stepwise. Therefore, from the viewpoint of the spectators, he may feel the overlapping of the image of the new frame on that of the old frame, so as to cause the blurring of the image outlines and produce the phenomenon of the so-called “after-image”.
When utilizing an LCD display as the displaying device of a personal computer, this after-image phenomenon is not evident and usually will not be noticed, since the images it displays are static images that are displayed most of the time. However, when utilizing this LCD displaying device for use as a television, the problem of slower LCD gray scale optical response will be more pronounced, since almost all the television programs utilize dynamic images. Therefore, the image displaying effectiveness of the conventional LCD television is evidently inferior to that of a CRT television.
In order to eliminate the above-mentioned after-image caused by slow optical response of the LCD display device, and the resulting image outline blurring phenomenon, currently most LCD television manufacturers try to convert the “hold type” display of the LCD displaying device into the simulated (or pseudo) impulse type LCD displaying device similar to that of the CRT displaying device, by means of a so-called “overdrive” technology, with its image only occupying a portion of the frame period according to the optical response as shown in curve (b) in FIG. 1, namely, the image is not displayed during a portion of each frame period.
The method utilized in this technology is a kind of “overdrive” method. It applies a voltage (for example code 200) which is much higher than the originally set target voltage (for example code 120) to the liquid crystal material, so as to expedite and accelerate the response speed of the liquid crystal molecules, and accelerating them to reach the predetermined optical response value, and as such shortening the liquid crystal gray level response time to less than one frame period, as shown in curve (b) in FIG. 1.
However, even the LCD display device made with this kind of overdrive technology is able to shorten its gray level response time to less than and within one frame period; yet due to the intrinsic property of the liquid crystal, the generation of the optical response is slow and so is its decline. Therefore, the image overlapping and the image outlines blurring phenomenon of the “after-image” for the images displayed still can not be eliminated completely.
In order to completely eliminate the “after-image”, presently there are three methods adopted by the prior art, which are listed as follows:
(1) to write black data or black images into the frame in the remaining portion of that frame period after the original formal image is displayed;
(2) to shut off the backlight, for example, the blink light method as published by Hitachi;
(3) the combination of the above methods (1) and (2), namely, both write in black image and shut off the backlight.
And in the following we will explain their respective drawbacks and limitations in detail.
First, referring to FIGS. 2A-2C, which illustrate the methods adopted by the prior art in simulating the impulse-type CRT display using an LCD display device, the images displayed by the liquid crystal display of the prior art are composed of a series of frames 1, 2, 3, and 4. The method utilized is to insert the complete black frames 11, 12, and 13 between frames 1, 2; frames 2, 3; and frames 3, 4 so as to achieve the simulation of an impulse-type CRT display, and at the same time the backlight source at time points 14-20 corresponding to the time points of the above-mentioned frames are all in the illumination state.
Next, we are going to explain the second method of the prior art. Please refer to FIG. 2B. At this time, the images displayed by the LCD display consist of the sequentially displayed frames 1-7. The second method is performed by shutting off the backlight source at time points 22, 24, and 26 which are corresponding to the time points of frames 2, 4, and 6, and the backlight source at time points 21, 23, 25, and 27 which are corresponding to time points of frames 1, 3, 5, 7, and 9 which are in the illumination state, and in this manner, achieving the simulation of the impulse-type CRT display using an LCD display and eliminating the “after-image”.
And then next, we are going to explain the third method of the prior art. Please refer to FIG. 2C, which indicates that the images displayed by the LCD display are composed of a series of sequentially displayed frames 1-4. The method is carried out by inserting the complete black frames 11, 12, and 13 between frames 1, 2; frames 2, 3; and frames 3, 4 respectively, and by putting the backlight source at time points 22, 24, and 26 corresponding to those of frames 11, 12, and 13 into the shut-off state, and by putting the backlight source at other time points corresponding to those of frames 1, 2, 3, and 4 into the illumination state. And that is to say, the third method achieves the effect of simulating an impulse-type CRT display using an LCD display by inserting the complete black frames between frames 1, 2, 3, and 4, and at the same time utilizing this blink light mode of alternating illumination state and shut off state by means of shutting off the backlight source at the corresponding time points.
However, the three above-mentioned methods have their respective drawbacks and limitations.
First, the first method of inserting complete black frames between frames necessitates the addition of extra equipments, for example, a frequency doubling device. Supposing that the original image displaying speed is 60 frames/min, the application of this method then necessitates the addition of the frequency doubling device to increase the image displaying speed to 120 frames/min, and wherein half of the number are used for inserting those black frames. Therefore, the utilization of this method would increase the cost of the equipment. Besides, the doubling of the image display frequency leads to the increase of electric-magnetic interference (EMI), and these are the drawbacks and limitations of the first method of the prior art.
Next, the application of the second method also necessitates the addition of a frequency doubling device, so as to achieve the equivalent number of display frames/unit time. Since half of the frames displayed in the unit time correspond to the backlight shut-off state, and cannot be displayed as visible images, the second method will increase the cost of the equipment, and it will also cause the increase of EMI. In addition, it requires the addition of extra equipment so as to make the backlight source blink, and therefore, it further increases the cost of this method. And these are the drawbacks and limitations of the second method of the prior art.
And next, the third method of the prior art is a combination of the above two methods, namely, inserting the black frames and blinking the backlight modules. As such, the drawbacks and limitations of the third method include those of the above two methods. Therefore, it is not satisfactory either.
In addition, in the above first and second methods, since the characteristics and speeds of optical response of different liquid crystal materials are different, the method of inserting black frames is not suitable for certain liquid crystal materials. Because for certain liquid crystal materials, their optical responses are fast from brightness to dark, and are slow from dark to brightness; but for other liquid crystal materials, their optical responses are slow from brightness to dark, and are fast from dark to brightness. Therefore, the effectiveness of inserting black frames at equal time intervals in simulating the impulse-type CRT display is not ideal and thus not satisfying, and in certain circumstances it is even not suitable for application. And it cannot achieve the simulation of the CRT display using an LCD display, and it is not able to achieve the effectiveness of eliminating the “after image” either.
In view of the various above mentioned drawbacks and limitations of the prior art, the inventor of the present case dedicates all his talent, ingenuity, knowledge and experience in this field to the related research, development, experiment, and improvement, so as to bring about the realization of the present invention.